The given invention is related to the field of metallurgy. More particularly, the invention relates to methods of preparing titanium alloys working with the lamellar structure. This invention is useful for making large semi-finished parts by working them with pressure to subsequently make finished products of different shapes. The large section parts may be utilized in aerospace industries, for example, disks, jet-engine blades, and airframe structures.
Titanium alloys are hard to deform materials. Titanium alloys with microstructures of fine grains have better plasticity than those with coarse grains. The need to increase the plasticity of titanium alloys results in the need to develop methods of making titanium alloy microstructures with grain sizes less than about 10 micrometers. Such a microstructure can be obtained in small section semi-finished products, for example, in hot-rolled bars with diameters not exceeding 60 millimeters. The microstructure of semi-finished products is coarser in large section sizes. Most semi-finished products have a coarse lamellar microstructure with crystallographic and metallographic texture having a grain size larger than 100 micrometers.
Grain coarsening in titanium alloys results in unsatisfactory mechanical properties. A noteworthy feature of a coarse-grained lamellar microstructure is that it has a high degree of structural heterogeneity which results in combination of low tensile strength and lower plasticity, low fatigue properties, and significant scatter in mechanical properties. It is not possible to make make the coarse-grain lamellar microstructure into a fine-grain microstructure by heat treatment. On the otherhand, it is possible to coarsen fine-grain microstructure of titanium alloys at the temperatures of single beta phase existence and at the temperatures in the two phase alpha and beta regions.
Producing a homogeneous fine-grain microstructure in titanium alloys would improve the technological properties during the thermomechanical processing due to reduction of stresses required for plastic flow. Also, the superior mechanical properties of semi-finished products would be provided, as well as superior mechanical properties after heat treatment.
It is known that it is possible to obtain a fine-grain microstructure in titanium alloys after rapid cooling from the molten state. Because of the low heat and temperature conductivity of titanium alloys, it is impossible to use this method for industrial titanium ingots made in accordance with the modern technologies. This method is used in powder metallurgy.
It is also known that in powder metallurgy it is possible to obtain a fine-grain microstructure due to the use of fine powder (about 50-100 micrometers) and a high solidification rate. One of the most significant disadvantages of powder metallurgy is the need to consolidate the powder. This limits the dimensions and size of the products, increases time, labor input, and the cost of the semi-finished product. Also, there is significant grain growth during consolidation due to the high temperatures of consolidation. Impurities from the powder particle surfaces reduce properties in comparison with the alloys made in accordance with traditional methods.
The conventional method of working titanium alloys, such as gatorizing, is known. It includes initial working at temperatures about 56xc2x0 C. lower than the recrystallization temperature, with subsequent heating and working (pressing/forging) of the alloys at the recrystallization temperature. The recrystallization temperature is the temperature that the metal starts to deform. The formation of fine-grain microstructure is achieved by means of recrystallization of a work-hardened material during subsequent working. The above-mentioned method of microstructure refinement can be applied mainly for semi-finished products which had been previously intensely hot-worked in the alpha and beta regions. An elongated microstructure and strong texture are formed in these worked titanium alloys, which lead to significant allotropy of the mechanical properties.
Thus, the prior art shows that the methods to manufacture large section titanium semi-finished products with uniform fine-grain microstructure and mechanical properties have not been achieved. There is a need to have methods to process titanium alloys that have substantially homogeneous fine-grain microstructure and mechanical properties throughout the workpiece.
This invention satisfies the above-mentioned need by providing a method of manufacturing titanium alloys in large-section semi-finished product form with controlled microstructure in microgram and subcrystalline states of aggregation, with reduced metallographic texture, to achieve the desired combination of mechanical properties in the titanium alloy product.
In one embodiment of the invention there is a method for preparing a titanium alloy article, distinguished by having a substantially controlled homogeneous fine grain microstructure, comprising the steps of: (1) starting with a titanium alloy article having an initial grain size (do); (2) selecting a final homogeneous fine grain size (dK) to be achieved in the titanium alloy article; (3) plotting a curve of the relationship between a recrystallized grain size (d) for the titanium alloy on the y-axis versus a strain temperature (T) for said alloy on the x-axis, between a range of 400xc2x0 C. and a temperature of complete polymorphous transformation (TCPT), in accordance with the relationship d=f(T); (4) locating an area T* on the strain temperature axis to divide the temperature axis into two zones comprising a first zone 400xc2x0 C. to T*, and a second zone T* to TCPT, where said. T* is located by first calculating a corresponding recrystallization grain size (d*) on the y-axis, where d* is logarithmically related to the initial grain size do; (5) further locating on the curve the final grain size (dK) on the y-axis and then a corresponding strain temperature (TK) on the x-axis; (6) determining the heating and deforming stage or stages to process the article based on TK, where for TCPT greater than TK greater than T*, there is at least one heat and deforming stage to obtain the final grain size dK, and where TK less than T*, there are at least two heat and deforming stages where each heat and deforming stage occurs for a sufficient amount of time to reduce the grain size of the titanium alloy article [about 2 to 10 times] until the final grain size dK is obtained; (7) then heating and deforming the titanium alloy article in accordance with the determined number of heat and deforming stages to achieve dK, where each heat and deforming stage has at least one heating and deforming step and one cooling step, where said heat and deforming stage occurs for a sufficient period of time to reduce the grain size of the titanium alloy article, and where the deformation of the titanium alloy article is in a substantially controlled manner during each heat and deforming stage at a rate of strain to achieve the desired grain size of the heat and deforming stage, where the true strain during the deformation is greater than or equal to 0.6 for each heat and deforming stage, and where said subsequent cooling is controlled at a temperature below the heat and deforming stage temperature at a cooling rate for substantially maintaining the reduced grain size obtained during the heat and deforming stage; and (8) repeating step (7) until the final substantially controlled homogeneous grain size dK is obtained in the article having substantially homogeneous mechanical properties.
The final fine grain size is less than or equal to about 15 micrometers. A fine grain size is defined as grains having less than or equal to about 15 micrometers diameter. More narrowly, a preferred fine grain is less than 5 micrometers diameter. Conversely, large grains are greater than 15 micrometers diameter. When there is at least two heat and deforming stages, each heat and deforming stage occurs for a sufficient amount of time to reduce the grain size of the titanium alloy article about 2 to 10 times until the final grain size, dK, is obtained.
Another embodiment of the invention is a method of making a substantially controlled homogeneous fine grain microstructure in a titanium alloy article, comprising the steps of: heating and deforming at a predetermined heat and deforming stage temperature at or below a temperature of complete polymorphous transformation where the titanium alloy article has sufficient ductility and a starting grain size, said heat and deforming stage contains at least one heat and deforming step and at least one cooling step, and where said heat and deforming stage is for a sufficient amount of time to reduce the grain size from the starting grain size at the beginning of the heat and deforming stage to a reduced grain size at the end of the heat and deforming stage, where deforming the titanium alloy is in a controlled manner at a rate of strain to achieve the desired grain size of the heat and deforming stage, where the true strain during the deformation is greater than or equal to 0.6 for each heat and deforming stage and where the cooling step is performed after the heat and deforming step at a temperature below the heat and deforming stage temperature, in a controlled manner at a cooling rate to substantially maintain the reduced grain size obtained during the heat and deforming stage; and continuing to heat and deform then cool the titanium alloy article, at lower heat and deforming stage temperatures than the previous heat and deforming stage temperature, so a reduction of grain size is achieved in subsequent heat and deforming stages until a final controlled grain size with controlled mechanical properties is obtained in the titanium alloy article.
Still another embodiment of the invention provides methods of making large section semi-finished products without any limits in size and shape. In this invention, semi-finished product is defined as a material form, such as a bar, plate or billet, that is supplied for further processing. For example, a bar is a semi-finished product from which bolts, pins, etc., are manufactured. As another example, a billet is a semi-finished product from which aircraft disk forgings are made. The term large section is relative to the part being semi-finished. For instance, a titanium alloy billet greater than about 4 inches could be termed large, disk forgings greater than about 30 inches diameter or 6 inches thickness are large. Also, when referring to titanium intermetallic based compositions such as those based on Ti3Al (alpha 2), Ti2AlNb (orthorhombic), or TiAl (gamma), a large section would be much smaller, since intermetallic materials are typically harder to process. A gamma or orthorhombic forging greater than about 12 inches could be termed large. The term Lamellar structure refers to alpha phase (hexagonal crystal structure) titanium arranged in plates. Often, plates share a common crystallographic orientation and are termed a xe2x80x9ccolonyxe2x80x9d.